Facing a broadband era, a WDM (Wavelength Division Multiplexing) transmission system able to communicate in a plurality of optical wavelengths is introduced progressively in order to utilize an optical fiber efficiently. In recent years, utilization of a DWDM device (the Dense Wavelength Division Multiplexer) able to multiplex dozens of optical wavelengths and perform transmission with further high speed also expands widely. Each WDM transmission system requires light sources corresponding to each optical wavelength and the required number thereof is rapidly increasing with higher multiplexing. Further in these days, a study for commercializing a ROADM (Reconfigurable Optical Add/Drop Multiplexers) which adds/drops arbitrary wavelength in each node advances. With introducing the system, optical path switching based on wavelength changing can be realized in addition to the enhancement of transmission capacity based on wavelength multiplexing and the flexibility of an optical network exceedingly expands.
As a light source for the WDM transmission system, a distributed feedback type semiconductor laser (a DFB-LD; Distributed FeedBack Laser Diode) oscillating in a single axial mode has been widely used until now because of the usability and high reliability thereof. In the DFB-LD, a diffraction grating having a depth of approximately 30 nm is formed in the entire area of a resonator and realizes stable oscillation in a single axial mode at a wavelength corresponding to a product of the diffraction grating period and double of the equivalent refractive index. Since the DFB-LD cannot be tuned over broad range of oscillation wavelength regardless of the stable oscillation in the single axial mode, the WDM transmission system is configured by generally using products different only in the wavelength in every ITU grid. For this reason, it is required to use different products for every wavelength, and that results in increasing of cost for stock control and keeping surplus of stock for dealing with a failure. Furthermore, in the ROADM that changes an optical path by changing a wavelength, since a variable width thereof is limited to a range of wavelength changing depending on the temperature, that is, approximately 3 nm when a normal DFB-LD is used, it becomes difficult to configure an optical network which utilizes features of the ROADM positively using resource of wavelength.
To overcome the problems of the present DFB-LD and realize the single axial mode oscillation over broad range of wavelength, the wavelength-tunable laser is energetically studied. The wavelength-tunable laser is classified broadly into two types. In one type, a wavelength-tunable mechanism is formed into a same element with a laser resonator. In the other type, the wavelength-tunable mechanism is formed outside the element. As the former, many structures are proposed, such as a DBR-LD (Distributed Bragg Reflector Laser Diode) where an emission area and a distributed reflection area are arranged with being separated in a same element, which is shown in FIG. 1, a Sampled-grating-DBR-LD which periodically changes diffraction grating period further and sandwiching an emission area therebetween, which is shown in FIG. 2, and an SSG (Super Structure Grating)-DBR-LD similar to this, which is shown in FIG. 3. At first, the tuning range of the wavelength of the DBR-LD was limited to approximately 10 nm at most. However, the later proposed Sampled-grating-DBR-LD realizes a wavelength-tunable operation over 100 nm and a quasi-continuous wavelength-tunable operation in 40 nm by skillfully utilizing the vernier effect being specific to the structure.
As the latter wavelength-tunable light source in which the wavelength-tunable mechanism is arranged outside the device, a method for arranging a diffraction grating outside the element as shown in FIG. 4, precisely adjusting an angle and a distance thereof, and performing a wavelength-tunable operation is proposed.
A structure for realizing a wavelength-tunable light source by configuring an optical resonator with using a PLC (Planar Lightwave Circuit) and by directly mounting an LD or an SOA (Semiconductor Optical Amplifier) on the PLC is also proposed. FIG. 5 shows a structure realizing a wavelength-tunable light source by combining a ring resonator and an SOA. The ring resonator composed of the PLC is characterized in that the lengths of the circumferences are slightly different to each other. The differences of the length of the circumferences generate the vernier effect to realize the wavelength-tunable operation in broad wavelength range.
In addition to these wavelength-tunable laser elements, research and development for a configuration in which a modulator is added in the same module are presently advanced. FIG. 6 shows an example of this. A Mach-Zehnder modulator is integrated monolithically on an output side of the above mentioned Sampled-grating-DBR-LD to generate high-speed and low-chirping modulation signals enabling long distance and large capacity optical communication. By using this integrated element, both of a wavelength-tuning and a modulation can be implemented by using a very compact module. Herewith, substantial miniaturization of a wavelength-tunable transponder module can be realized.
Some examples of documents describing related techniques will be shown below. Japanese Laid-Open Patent Application JP-P2006-278769A describes a wavelength-tunable laser which includes a multiple-ring resonator composed of ring waveguides having optical lengths to each other.
Japanese Laid-Open Patent Application JP-A-Heisei, 09-80497 describes a laser beam generator which realizes the increase of wavelength conversion light power by enabling the enhancement of the combining efficiency of a semiconductor laser with an optical waveguide through changing a polarized wave direction of an oscillating mode of the semiconductor laser as a main wave light source.
Japanese Laid-Open Patent Application JP-A-Heisei, 11-26875 describes an optical combined module introduced by mounting all emitting/receiving elements and optical elements on a substrate and by using a passive alignment method for securing the optical elements.
Japanese Laid-Open Patent Application JP-P2000-231041A describes an alignment technique and will be referred later.